Usage of joint rating functions for seismic phase association and event location Asming V.E., Prokudina A.V., Nakhshina L.P. Kola Regional Seismological Centre Russia

MAIN IDEA When a seismic phase is detected a set of azimuth-dependent rating functions for the phase can be computed : a) Polarization functions P P (α) – estimation of hypothesis that the phase is P coming from α; P S (α) – estimation of hypothesis that the phase is S coming from α; b) Beamforming (sum of shifted array channels) B(α,V) – estimation of hypothesis that the phase is coming from α with velocity V The functions are usually used to determine phase types. We propose to use combinations of the function for several phases simultaneously to check hypothesis that the phases are of the same seismic event The approach can be useful for single stations (3C and arrays) as well as for sparse networks

Joint analysis of couples of phases detected at the same station The idea: Detect some phases (say, using STA/LTA) For each couple of phases check a hypothesis that the 1st one is P and the 2nd one is S from the same event. If the estimation is greater than a threshold then locate the event by the backazimuth and S-P time difference Implementation: Joint polarization analysis Joint beamforming Usage some penalties and Bayesian belief networks to take into account recording (envelope) shapes

Joint polarization analysis P S Red line : R(α) is normalized horizontal motion Blue line : C z (α) is correlation between horizontal and vertical motion Estimations of phase kinds : P P (  )= (1 + R(  ))  (1+C Z (  ))/4 P S (  )= (1 + R(  +90  ))  (1-  C Z (  +90  )  )/2 Estimation of the hypothesis that phase A is P and B is S from the same event: where Penalty is some functional dependent on the phases Joint estimation for backazimuth α

Joint beamforming for an array Checking the hypothesis that the 1 st phase is P and the 2 nd one is S from the same event by an array sensors. P S ΔTpΔTp ΔT s = ΔT p ·(V p /V s ) upper layer The idea is to make beamforming simultaneously for P and S fragments and use related time delays for P and S.

Joint beamforming for an array The estimation of the hypothesis that the 1 st phase (A) is P and the second one (B) is S from the same event coming from the backazimuth α with apparent P velocity V p is: Where (t A1,t A2 ) is time interval for P candidate, (t B1,t B2 ) is for S candidate; Z i (t) : samples of i-th sensor Δt i (α,V,V upper ) is time difference between arrivals of a wave coming from backazimuth α with velocity V to the array centre and to i-th sensor. It depends also on upper layer velocity V upper if we take into account sensors elevations R=V p upper /V s upper : P and S velocities ratio under the array If the array includes a 3-component sensor we can multiply JB(α,V p ) by the polarization estimation P PASB (α). The final estimation is Rating(α,V p )=P PASB (α)·JB(α,V p )

For the algorithm it is important to know V p and V p /V s under an array We have selected several events, picked manually P and S phases and computed their compatibility estimations for different variants of V p upper and (V p /V s ) upper: Storfjorden events. Maximum at Vp=5.5 km/sec and Vp/Vs=1.8 SPI array, Spitsbergen Events from North-West. Maximum is not realistic! Vp/Vs~1 ! SPI array, Spitsbergen

Joint analysis of couples can screen out non-realistic candidates to be phases P candidate Centre of envelope S candidate Penalties dependent on envelope shape and position of phases inside envelope can be implemented to screen out non-realistic couples. In this case the following penalties are used to decrease the rating: Due to presence of a phase before the P candidate; Due to large time difference between the envelope centre and the S candidate; Due to huge amplitude ratio of the P and S candidates; Such penalties and/or rating functions can be combined in a more strict manner using probabilistic approach based on Bayesian belief networks. Dist to the centre P ampl S ampl Phase before P

An example of Bayesian belief network analyzing couples of phases Linear polarization of 1 st phase Yes/No V apparent 1 st phase>VpMin Yes/No Is the 1 st phase P-wave ? Yes/No Is compatible phase before the 1 st ? Yes/No Is this P-wave of event beginning ? Yes/No 1 st and 2 nd phases are compatible by polarization Yes/No 1 st and 2 nd phases are compatible by joint beamforming Yes/No Phases are connected in envelope Yes/No The couple is P/S of the same event Yes/No Final decision

Usage of the joint beamforming algorithm for Storfjorden seismicity monitoring (UDL program) STA-LTA detector for generalized envelopes; P and S association in time interval 9-15 sec (to avoid false associations for different events in the same area); GBF UDL Manual analysis has shown that at least 95% of the events are true. Linear slope of the frequency-magnitude curve for M>-0.2 leads us to conclusion that we detect the most part of the events with M>-0.2. SPI data had been processed since 2008 Storfjorden is a seismically active zone at distances km from SPI array

Near real-time Storfjorden monitoring (since )

Joint beamforming by a couple of arrays (idea) Location of Zapolyarny explosion by two arrays : AP0 (Apatity) and ARC (Norway). When P onsets are picked the line can be drawn on which the event occurred (P-P). Distances to APA and ARC are about the same ( km) so we can expect apparent velocities at the arrays to be the same.

Example of joint beamforming by couple of phases : S(AP0) and S(ARC) for Zapolyarny explosion X axis : length (L) along S-S line Y axis : apparent velocity (km/sec) (the same for both arrays) Red points : uncertainty area (B>0.99 Bmax) Maximizing function : Where l is length along S-S line, V is apparent velocity, α is backazimuth to an array dependent on l. B is amplitude of an array beam (sum of time-shifted channels) In more complicated cases V apparent can be calculated using travel-time model.

Perspectives of joint data processing The European Arctic is area with a very sparse seismic network. But huge amount of seismic events with complicated shapes of wave forms occur here. Every known algorithm of detection and location makes a lot of false alarms. It is difficult to obtain a realistic picture of local seismicity. We suppose to make a new data processing system based on the following principles: A collection of algorithms where each one estimates probability of some fact (for example, that a part of recording contains an onset, that an onset is P wave, that a part of envelope corresponds to a seismic event etc.). The algorithms could be very different (ordinary STA/LTA, envelope-based, neural networks etc.) but operate with standard data types; A system that calls the algorithms according to absence or presence of some data types (3C data, array data, envelopes etc.) A Bayesian belief network that makes a final decision about processed data sets based on results of work of the algorithms (does a data set contain a seismic event or not, are onsets found, what are the waves etc.)

Thank You for attention !